ECEN474: (Analog) VLSI Circuit Design Fall 2012ece.tamu.edu/~spalermo/ecen474/lecture18_ee474_ota_cmfb.pdf · Sam Palermo Analog & Mixed-Signal Center Texas A&M University ECEN474:

Sam Palermo Analog & Mixed-Signal Center

Texas A&M University

ECEN474: (Analog) VLSI Circuit Design Fall 2012

Lecture 18: OTA CMFB Examples

Announcements

• No class on Monday • Preliminary report still due Monday (11/19)

• If you are doing anything not on the list, give me a brief description as soon as possible

• We will have class on Wednesday (11/21) • Project Extra Credit

• Potential for 20% extra credit if you layout a key block • Report on Dec. 4 doesn’t have to have layout • For extra credit, an updated report can be turned in during the

presentation time (Dec. 10) with layout results • The post-layout performance is the only thing that will be considered

relative to the original report, i.e. no major circuit changes from the original report

2

Agenda

• Multi-OTA stages CMFB • OTA-C filter w/ CMFB example

3

TAMU-ECEN-474 Jose Silva-Martinez_08

- 4 -

2 IB

IB+icmfb

GND

VC

IB

IB

IB IB

GND

IB

VC

CMFB is required for Differential Structures

CMFB Requirements: Fixes the OTA output (low offset) ==> High dc loop gain Reduction of common-mode noise==> Large Bandwidth

CMFB CMFB

Rg1g

Gm

mm +=

R R mm gG =

GND

CMFB icmfb = gcmfb(v01+v02-Vref)

IB+icmfb IB+icmfb


- 5 -

IB

VB1

IB IB

VREF IB

Common-mode loop gain = AV Gmp RL 3 poles in the CMFB loop. Loop stability requires AV Gmp / CL < ωp2 @ VC, ωp3 @ VB1

IB

IB IB

IB

VB2

VDD

VSS

VC

Efficient CMFB for Differential Pair Based OTAs

+ - AV


- 6 -

IB IB

Sensitive to supply noise and common-mode input signals

IB

IB IB

IB

VDD

VSS

V1+v1 V2+v2

From CMFB

to next stage

IB

GND

VDD

VSS

From CMFB

to next stage

v2 v1

MC

M1

R1 R1

M1

Pseudo-Differential OTAs with Source Degeneration

Little sensitive to supply noise and Common-mode noise


- 7 -

IB

IB IB

IB

CMFB

IB

IB IB

IB

VDD

VSS

IB

GND

M1 M1 R1 R1

M1

Efficient CMFB for Pseudo-Differential OTAs

common-mode detector


- 8 -

OTA based on complementary differential pairs

-v1 v1v0 -v0

VCP

VCN

CLCL

M1 M1

M3 M3

M4 M4

M2 M2

VDD

VSS

Efficient OTA based on linear complementary differential pairs

Linear circuit due to source degeneration M3 and M4

Suitable for fast applications

11 22

2

31

1

++

+=

Mm

m

Mm

mm Rg

gRggG


- 9 -

Filter is based on Biquadratic Cells: Biquad Realization in Gm-C topology

!!! Fast CMFB is required

vi-

vi+

vo-

vo++ -

- +

+ -

- +

+ -

- +

+ -

- +

gm1 gm2 gm1

2C2

2C2

2C1

2C1

gm1

f0 (MHz) Gm1 (mA/V)

Gm2 (mA/V)

Biquad 1 537.6 5.4 9.6 Biquad 2 793.2 5.4 5.07


- 10 -

Time Domain characterization of the CMFB Common-mode characterization using common-mode current pulses One CMFB circuit per pole •Pulse response of the

CMFB •Phase margin is better than 45 degrees

CMFB

icm

icm

vi-

vi+

vo-

vo++ -

- +

+ -

- +

+ -

- +

+ -

- +

CMFB


- 11 -

-v1VREF

Previous OTA CMFB Next OTA

v1

A B

C

DE

v03 v04v01 v02

OTA with Class AB Common-mode Feedback

• Common-mode signal is detected at next stage • Class AB error amplifier is used • 5 non-dominant poles at A~E • 2 LHP zeros at A and C (Helpful in BW extension)

• Most important non-dominant pole at D


- 12 -

Optimized Class AB Common-mode Feedback

• Most important non-dominant pole at B

v03 v04-v1 v1v01 v02

VREF

Previous OTA CMFB Next OTA

A A'B

C

M1M3

M6

M5E'

E

• Class AB error amplifier is used • 4 non-dominant poles at A~E • 2 LHP zeros at A and C (Helpful in BW extension) • Node D was eliminated


- 13 -

+

+

+

+

≅

1m

D

03

B

L

L

L6m

5m

5m

VCMFB

g

sC1

g

sC1

g

sC1

gg

g21

g

A

Analysis of Class AB Common-mode Feedback

•2 pole-zero pairs (A and C) are very close to each other

•More stable CMFB

CMFB can be simplified taking advantage of circuit’s symmetry


- 14 -

Remarks • DC operating points for high impedances are difficult to fix • Fully differential amplifiers with high output impedance nodes must use common-mode feedback circuits . • Common mode circuits can fix the DC operating points as well as minimize the common mode output components. • Low voltage constraints impose optimal bias conditions at both the input and output ports of an amplifier. • Common mode circuits for LV should be used both at the input and output

Next Time

• Analog Applications • OTA-C Filters • Variable-Gain Amplifiers • Switch-Cap Filters, Broadband Amplifiers

• Output Stages • Bandgap Reference Circuits • Distortion

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Documents

ECEN474: (Analog) VLSI Circuit Design Fall 2012ece.tamu.edu/~spalermo/ecen474/lecture18_ee474_ota_cmfb.pdf · Sam Palermo Analog & Mixed-Signal Center Texas A&M University ECEN474: